Anyone else running a split-charge relay alongside a DC-DC charger? Worth it or just redundant?

So I've been scratching my head over my setup for a few weeks now. I've got a Renogy 40A DC-DC charger fitted in my Transit-based van, pulling from the alternator into a 200Ah LiFePO4 leisure battery. Works a treat on long motorway runs, battery's topped up well before I reach site. The thing is, I've still got the old split-charge relay from the previous owner sitting there doing nothing, and I'm wondering whether there's any point keeping it wired in at all.

From what I understand, the DC-DC charger already handles the voltage conversion and protects the starter battery from being dragged down — which is essentially what the relay was doing in the simpler lead-acid days. Running both simultaneously feels like it might actually cause problems rather than solve them, especially with the lithium wanting a proper CC/CV charge profile rather than just a direct alternator connection.

Has anyone actually tested whether leaving the relay active alongside a B2B charger causes any grief? I've heard a few people say it can confuse the alternator or create weird voltage spikes, but I've not found anything definitive. Tempted to just yank the relay out entirely and keep things clean, but don't want to pull the trigger if there's a genuine reason to keep it.

Hey @Spider85, running both is largely redundant in your case. The DC-DC charger is doing the proper job already — it's isolating your starter battery, managing the charge profile correctly for LiFePO4, and protecting your alternator from the battery's aggressive acceptance rate. A split-charge relay on top of that adds complexity without real benefit, and could actually cause headaches if the relay closes at the wrong moment and bypasses your DC-DC charger's protection circuitry.

Where a relay does still make sense is if you wanted near-instant bulk transfer for something like a large AGM bank, but that's not your situation.

Stick with what you've got, keep it clean and simple. The Renogy 40A is a solid unit for a Transit setup — I ran the same one for two years without a grumble. 👍

@Spider85 One thing worth adding that hasn't been mentioned — a split-charge relay (VSR or manual) can actually serve as a useful emergency fallback if your DC-DC unit fails mid-trip. I learnt this the hard way when my Victron Orion-Tr Smart threw a fault on the A30 somewhere near Okehampton. Having a simple VSR wired in parallel meant I could still get some charge into the leisure bank to limp home.

That said, you'd want a LiFePO4-safe VSR or a manual override switch rather than a standard voltage-sensitive relay, which can behave oddly with lithium's flat discharge curve and potentially damage cells. For day-to-day use, the Renogy DC-DC is absolutely doing the heavy lifting — the relay is just insurance, not a primary strategy.

@Spider85 Just to build on what @PaulaFisher91 was getting at — another angle worth considering is failover. If your DC-DC charger ever packs up on a long trip, a simple VSR wired in parallel gives you a basic fallback to at least keep some charge going into the leisure battery. Not ideal long-term for LiFePO4, but it could save your bacon miles from anywhere.

That said, on a 40A Renogy you're already getting proper voltage and current regulation, so day-to-day the relay adds nothing meaningful. I'd only bother if the wiring's already partially in place or you're doing a lot of remote touring where redundancy genuinely matters.

What's your typical usage pattern? Weekends on campsites or longer off-grid stints? That'd really shape whether it's worth the extra faff.

@JasonPhillips raises a fair point on failover, but honestly in practice I've never felt the need for belt-and-braces redundancy there.

What I would say from my own Transit build — the DC-DC charger alone is genuinely the right call for LiFePO4. The old VSR approach was designed for lead-acid behaviour; it'll just dump unregulated voltage straight into your lithium cells if it cuts in unexpectedly.

If anything, I'd focus on making sure your Renogy 40A is properly fused and your alternator isn't being hammered on long motorway runs. Mine got warm until I dialled the input current limit back slightly in the settings — worth checking if yours has that option.

Skip the split-charge relay unless you have a very specific use case. Two charging paths into a lithium bank without proper BMS coordination is asking for headaches.

@JubileeClipHero5 the "in practice" argument is fine until you're parked up in the Cairngorms in January with a dead DC-DC and no fallback.

One thing nobody's touched on yet — alternator protection. My Victron Orion-Tr Smart actively limits current draw and monitors battery voltage, which means my alternator isn't getting hammered during bulk charge. A straight VSR alongside it with a chunky LiFePO4 can absolutely punish older alternators; LFP will just drink whatever you throw at it until it's full.

In my tiny house build on a Sprinter, I ditched the split-charge entirely once I fitted the Orion. The DC-DC is the protection layer. Running both creates configuration complexity without meaningful redundancy unless you're deliberately isolating them with proper diode-based switching — which most van builds aren't.

What alternator are you running, @Spider85? That changes the calculation significantly.

@KangaoNomad makes a good point — I'm currently planning my Transit build and the failover question is exactly what's been nagging at me.

Quick question for the thread: if you did run both a split-charge relay and a DC-DC charger, would the relay even do anything useful while the DC-DC is active? My understanding is the DC-DC presents a controlled load to the alternator, but a relay would just throw the batteries in parallel — surely that could interfere with the Victron (or Renogy in @Spider85's case) doing its thing?

Is there a wiring approach where the relay only kicks in as a genuine fallback rather than running simultaneously? Or is that getting too complicated to be worth the effort on a van build?